1. Field of the Invention
The invention pertains to a device for dynamically load testing a sample that comprises a bearing, to which the sample can be directly or indirectly connected in a detachable manner, as well as a counter-bearing that can be effectively connected to the sample by means of at least one actuator element such that the at least one actuator element introduces dynamic mechanical loads into the sample which act along a load path that is directed between the bearing and the counter-bearing and extends through the sample, wherein the at least one actuator element features a multifunctional solid body conversion material system that undergoes deformations due to the supply of energy, and wherein said deformations are the cause or at least one of the causes for the mechanical loads occurring within the sample.
2. Description of the Prior Art
The constantly increasing quality requirements with respect to technical modules and components are one of the major reasons why it needs to be ensured that the applicable quality and safety standards are always observed. Numerous different test devices and test methods are known for realizing the quality assurance and the quality control, wherein specific variations consist of devices for carrying out dynamic load tests, in which technical systems, modules and components or even simple material samples can be tested with respect to their mechanical load carrying capacity. For example, such dynamic load tests make it possible to obtain information on material/component properties, material/component fatigue and crack formation or crack growth per load alternation acting upon the respective component, as well as to obtain information with respect to the dynamic structural behavior.
So-called pulsators were developed for carrying out dynamic load tests on a sample, wherein the pulsators subject samples to a more or less sinusoidal load alternation. For this purpose, the sample is fixed in a suitable clamping device on two opposite sample regions that define the load path, along which the sample is subjected to dynamic compressive and/or tensile forces. In known test devices, the dynamically varying loads acting on or in the sample are generated with actuators that operate on the basis of servohydraulic, servopneumatic or servoelectric systems.
DE 198 20 322 A1 describes a servohydraulic resonance test machine that is representative for test machines with an actuator that operates on the servohydraulic principle. One side of a sample is detachably and rigidly fixed in a stable loading frame by means of a suitably designed interface, wherein the other side of the sample is connected to the piston rod of a piston-cylinder unit by means of another interface. The cylinder unit is arranged stationary relative to the loading frame while the piston rod can be displaced relative to the loading frame in a controlled manner. The displacement of the piston rod takes place under the control of oil pressure.
DE 28 29 858 describes a pulsator, the actuator of which is based on an electromagnetic principle. The sample to be examined is clamped into a holding arrangement on one side and provided with a cap of magnetically conductive material on the other side, wherein this cap is arranged such that it is separated from an opposite electromagnetic pole by an air gap, and wherein an electromagnetic alternating field is applied between the opposite pole and the cap such that the cap connected to the sample is attracted to or repelled by the magnetic pole in the direction thereof in dependence on the orientation of the magnetic field.
The generation of alternating loads that can be induced in the respective sample to be examined is limited to a maximum frequency of approximately 100 Hz when using actuators that are based on servohydraulic, servopneumatic or servoelectric principles, namely due to their system design. Even in so-called high-frequency test systems or high-frequency resonant pulsators, for example, according to the test system described in above-cited German publication DE 28 29 858, the load alternation frequencies are typically limited to a maximum of 1000 Hz. Furthermore, high-frequency pulsators of this type only make it possible to generate monofrequent load signals such that they can only be used for so-called single-stage tests.
There also exist electrodynamic vibrators that make it possible to realize load alternation frequencies up to the acoustic range, but such vibrators that are also referred to as electrodynamic shakers are only able to induce modal or base-excited loads in the sample that do not act along a load path defined by at least two fixing points of the sample to be examined, but rather along a plane of vibration that simultaneously forms the supporting plane for the sample, in which the sample is connected to the shaker. An electrodynamic shaker of this type capable of generating load alternation frequencies up to 4000 Hz is sold, for example, by Forschungsgesellschaft Kraftfahrwesen mbH, Aachen.
New materials, structures, technical modules and components as well as systems are subject to requirements that make it necessary to utilize test systems that provide a great degree of freedom with respect to the test signals or load signals to be generated and substantially broaden the frequency range toward higher frequencies. Although the available options for controlling or regulating such highly dynamic processes represent immense challenges, they are no longer the central problem with respect to the technical devices due to the continuously increasing capacity of the computers used. On the contrary, what is currently needed is an option for coupling the technically generated load signals into the structures or samples to be tested along a predefined load path or flux in suitable form. It is therefore of the utmost importance to develop new test actuators or load application components, respectively.
GB 2 060 179 A describes a material testing device, in which one side of a material to be tested is clamped in a fixed counter-bearing and the other side is connected to an actuator unit that, in turn, is coupled to a frame connected to the fixed counter-bearing. The actuator unit has piezoelectric elements that are connected to one another in a stack-shaped manner. A corresponding electrical power supply U makes it possible for the piezoelectric elements 22 to generate high-frequency vibrations that are introduced into the sample for load testing purposes. The utilization of such an actuator makes it possible to generate load alternation frequencies to the kilohertz range.
U.S. Pat. No. 3,842,662 describes a similarly designed device, in which the sample is subjected to high-frequency vibrations by means of piezoceramic elements. Hydraulic units are additionally provided for boosting the force of pressure, wherein the hydraulic units subject the sample to an additional mechanical load in hybrid form.
In a comparable arrangement for carrying out fatigue tests on a sample in accordance with publication GB 2 367 631 A, an actuator that has, among other things, with a piezoelectric or magnetostrictive material, serves for introducing forces into a sample to be examined and for generating resonant structural vibrations therein.